In the conventional photomechanical techniques, an original printing film of a halftone picture is formed from an original picture of a continuous tone (primarily, a photographic picture such as photographic monochrome or multicolor picture, photographic positive or negative picture or photographic reflection or transmission picture) by conducting halftone scanning by means of a photomechanical camera or performing both color separation and halftone scanning by means of an advanced photomechanical apparatus such as monochrome scanner, color scanner or total scanner. In a photomechanical process of the above sort, it is the routine practice to inspect and confirm the quality of a picture, which has been formed in an original printing film and is to be printed, by one to several proofs and then to make a final machine plate for printing the picture.
On the other hand, the so-called direct-plate photomechanical process in which the above-mentioned formation and proofs of an original printing film is omitted has found actual utility only in extremely-limited cases, for example, where an original does not contain any continuous-tone picture (for instance, a photographic halftone reflection picture contained in an original is considered to be a sort of line drawing in the present field of art) or in the case of newspapers or the like in which the quality of pictures in the prints may be sacrificed for shortening the time required for the photomechanical plate-making work.
If the direct-plate photomechanical process practised only in extremely-limited fields as described above is applied to an original which contains one or more photographic continuous-tone pictures and is to be printed (incidentally, such an original usually contains one or more photographic continuous-tone pictures), it is extremely difficult to stably produce generally-acceptable standard.
Except for the extremely-limited cases mentioned above, the direct-plate photomechanical process has therefore not been recognized as one of photomechanical processes.
Namely, the formation of an original printing film and the step of proof are indispensable without exception for all photomechanical processes known to date, so that they cannot not be omitted.
The present inventors have heretofore made various proposals with a view toward improving the present direct-plate photomechanical process into a rational technical system applicable to an original which contains one or more usual photographic continuous-tone pictures. Namely, the present inventors have reviewed all steps, basic work, fundamental techniques and technical system involved in the overall process for forming from an original picture a picture to be printed, and have made various proposals in order to errors and defects included therein. For example, the present inventors proposed to improve the quality evaluation of a printed picture (i.e., to measure and control the two basic elements forming a printed picture "the size of each dot " and "the density of each printing ink" independently). Also proposed were the tone-rendering-first-ism and 4-point control method in the conversion of a picture, both, for the rationalization of color separation work (i.e., the present inventors proposed to include, as control points, at least four areas of H (the brightest point in the high-light area), S (the darkest part in the shadow area), M.sub.1 (intermediate density point of the picture) and M .sub.2 (the position where 50% dot area percentage is set). The present inventors also proposed to improve the system of an advanced photomechanical apparatus by using these techniques. Regarding these proposals, reference may be had, for example, to Japanese Paten Laid-Open Nos. 97101/1979, 98801/1979, 103101/1979, 115901/1979 and 115902/1979 and Japanese Patent Publication No. 11337/1987.
It was however unable to standardize, rationalize and systemtize the photomechanical technology to such an extent that the formation of an original printing film and proofing step mentioned above could be omitted in the photomechanical work.
It is however of urgent necessity for the present field of art to establish a direct-plate photomechanical technique which can substantially omit the above-described formation of a original printing film and proofing step and can also obtain printed pictures of stable quality.
The problem sought for solution in the present invention is concerned with the non-rational and non-scientific means applied to date for the conversion of the tone of a picture, namely, employed in the work for converting a continuous-tone picture into a halftone picture.
With a view toward establishing a direct-plate photomechanical technique also applicable to an original continuous-tone picture, the individual steps and work of the photomechanical process, the fundamental techniques applied to such steps and work, and the overall technical system of the process were studied in detail. As a result, the lack of establishment of the above-mentioned direct-plate photomechanical process by that time was found to be attributable to the fact that the tonal conversion of a picture, which is the first step of a process for forming a halftone picture to be printed from an original continuous-tone picture and is also the most important and basic step in color separation or the like, was dependent on non-scientific and non-rational means.
The present inventors then reached a belief that the establishment of a reliable direct-plate photomechanical technique would be difficult unless the above problem was solved.
Incidentally, the present tonal conversion for converting an original continuous-tone picture into a halftone picture to be printed is still performed on the basis of the experiences of perception of men such as designers of a photomechanical apparatus such as color scanner or its users or depending on a plurality of fixed given data. The tonal conversion is hence non-scientific and non-rational and lacks operational flexibility.
Incidentally, a standard work characteristic curve is employed as a standard or reference when a tonal conversion is effected by performing halftone scanning or both color separation and halftone scanning with advanced photomechanical equipment such as monochrome scanner or color scanner. This standard work characteristic curve is a characteristic curve of a halftone picture, which is to be used as a standard or reference upon performing halftone scanning or color separation. It is also called "a color-separation density characteristic curve", "a color-separation characteristic curve", "a halftone characteristic curve" or the like. As such a standard work characteristic curve, is used that determined by the manufacturer on the basis of its experiences and perception or information based on a limited number of fixed given conditions and stored in a memory of such an apparatus.
For the operator of the apparatus, the details and number of technical and operational choices in the photomechanical work have already been limited due to the non-rational and non-scientific knowledge of the design engineers of the apparatus. A limitation has also been imposed on the details and range of technical and operational measures for advanced and varied needs of the society for printed pictures and for nonstandard original pictures (pictures of color originals having quality other than the color original quality taken into consideration by the designers of photomechanical apparatus). Otherwise, the conversion of the tone of a picture is performed primarily on the basis of the experiences and perception of an operator.
Such apparatus include those designed in such a way that an operator sets a work standard characteristic curve as desired, scores it in a memory thereof and then uses same as needed. This procedure however requires complex basic experiments and is cumbersome work which requires a lot of time, labor and cost. Nevertheless, the new work standard characteristic curve cannot be set rationally and scientifically beyond the aforementioned boundary of the fundamental limitation. In the present state of the art, there are more and more cases in which a work standard characteristic curve must be set newly in a rational and scientific manner beyond the boundary of the fundamental limitation.
As has been described above, the present inventors have a view that the present advanced photomechanical apparatus such as monochrome scanners and color scanners basically do not make full use of the necessary and sufficient finding and knowledge on plate-making and printing, their design technology is composed of the theory of color separation based on the non-rational and non-systematic, photographic masking method, and the various improvements to the apparatus were basically only counter-measures based on the theory of color separation.
The present inventors have hence carried out an extensive investigation under the basic recognition that a technical means always capable of assuring the quality of a picture to be printed, namely, a rational and scientific technique for the tonal conversion of a picture has to be established in place of the conventional photomechanical process in order to establish a direct-plate photomechanical technique useful for the eternal rationalization of the photomechanical technology and its practice.
In the conventional design technology of scanners, color correction and color modification were given the priority and the tonal conversion of a picture was taken into secondary consideration, as described above, upon performing tonal conversion of a photographic original picture into a halftone picture in color separation work. In the tonal conversion of a picture, the conventional design technology has hence stuck with the 3-point control method which regards sufficient the control of the three points H, S and M.sub.1 without paying any attention to the control of the position where 50% dot area percentage required for the stabilization and improvement of the quality of a picture to be printed and for the conversion and control of gray balance of an intermediate tone. It is also the present state of the art that no sufficient consideration is give regarding how big effects a slightest change in area of each dot gives to the change in gradation and tone of a printed picture which is an assembly of numerous dots, although the development of such effects is an extremely important finding for plate-making and printing. The essential elements for expressing or visualizing a printed halftone picture, in which the control of the position for setting 50% dot area percentage and the control of the dot area are also neglected for the above-mentioned reason, are both "the area of each dot" and "the reflection density of an ink". As to the factor of the above-described "reflection density of the ink", the amount of an ink may be controlled within about .+-.20% in the case of Y, M and C (about .+-.40% in the case of a black plate) on a printing machine. Further, the visual sensation of a man has ability to easily distinguish as a density difference the difference of 1% in terms of "dot area" percentage. It is hence understood that the control of the area of each dot is extremely important in the formation or conversion of a halftone picture, when the above objective facts are taken into parallel consideration. It is therefore understood that when a conventional color separation technique is relied upon, a significant limitation lies as an obstacle to the theme of obtaining a printed picture having extremely high fidelity from an original picture such as photographic color picture.
In the conventional tonal conversion method of an original continuous-tone picture into a halftone picture to be printed, there was no means for correlating rationally and eternally the density at a desired control point on the continuous-tone picture with the dot area percentage of the dot at its corresponding control point on the halftone picture.
The present inventors have carried out an extensive investigation with a view toward breaking up the above-mentioned limitation of the conventional tonal conversion techniques which are non-rational and non-scientific. As a result, the present inventors have found a novel tonal conversion method for correlating the base density of a control point on the aforementioned original continuous-tone picture with the dot area percentage at the corresponding control point on the halftone picture. It has then been found that the novel, rational and scientific, tonal conversion method can be successfully applied to conventional photomechanical apparatus, can break up the limitation of the conventional color separation technology and can exhibit superb effects, leading to the completion of the present invention.